MTU

What is MTU?

MTU stands for Maximum Transmission Unit. It's the largest size packet (chunk of data) that can be sent over a network connection without being broken into smaller pieces. Think of it like the maximum box size a shipping company accepts—larger items need to be split into multiple boxes.

Most internet connections use an MTU of 1500 bytes, but VPNs and some ISPs use smaller sizes. When packets are too big, they must be fragmented (split up) or retransmitted, which can cause slowdowns or connection problems.

Why it matters

MTU mismatches are a sneaky cause of network problems. Everything might work fine until you try to load a specific website or use a VPN, and then things break. This happens because some data fits through fine, but larger packets get stuck.

Common symptoms of MTU problems: websites that partially load and then hang, VPN connections that connect but can't pass traffic, or some sites working while others don't. These issues are often misdiagnosed as server problems when MTU is actually the culprit.

What you can do

MTU issues are tricky because they're usually caused by a mismatch somewhere in the chain. Here's how to approach them:

• If you're using a VPN and some sites don't load properly, the VPN app's settings often have an MTU option—try lowering it to 1400
• Restart your router and modem—this clears buffers and can resolve temporary MTU issues
• If you have DSL internet, your ISP may have specific MTU requirements—call their support line if you're having persistent problems
• Try a different browser or device to see if the problem follows you—that helps narrow down whether it's your device or your network
• Contact your ISP if certain websites consistently fail to load fully—they can check if there's a configuration issue on their end

What Network Weather shows you

Network Weather detects your effective MTU and identifies potential MTU-related issues.